Change mastodon:setup to not call assets:precompile in docker (#13942)

It appears assets are built during image build, and they shouldn't need
to be rebuilt, since we now have reproducible builds.

lib/tasks/mastodon.rake

@@ -371,6 +371,7 @@ namespace :mastodon do
           end
         end
 
+        unless using_docker
           prompt.say "\n"
           prompt.say 'The final step is compiling CSS/JS assets.'
           prompt.say 'This may take a while and consume a lot of RAM.'
@@ -385,6 +386,7 @@ namespace :mastodon do
               prompt.say 'Done!'
             end
           end
+        end
 
         prompt.say "\n"
         prompt.ok 'All done! You can now power on the Mastodon server 🐘'